International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Volume 72 no. 1 / February 2021

Volume 72 no. 1 / February 2021

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Articles in this issue

  • Stratigraphical and sedimentological relationships of the Bolognano Formation (Oligocene–Miocene, Majella Mountain, Central Apennines, Italy) revealed by geological mapping and 3D visualizations

    Abstract: The characterization and comprehension of buried reservoirs receive remarkable benefits from detailed studies of outcropping analogues which help to define the architecture of the buried sedimentary units and their petrophysical features. In particular, modern 3D techniques of geological data analysis can better constrain the geological mapping process and reveal the geometry of the sedimentary units with complex lateral and vertical relationships. By means of the 3D Move software, we define the sedimentological and stratigraphical relationships between lithostratigraphic units of the Bolognano Formation, outcropping in the northernmost sector of the Majella Mountain (Central Apennines, Italy). The study area belongs to the Apulian carbonate platform and the Majella Mountain represents the northward outcropping portion of its margin. The sedimentary succession of the Majella Mountain consists of Upper Jurassic to upper Miocene limestone and dolostone deposits. In the investigated area, outcropping deposits mainly belong to the Oligo–Miocene Bolognano Formation characterized by five lithofacies associations and representing a carbonate ramp developed in a warm subtropical depositional environment within the oligophotic to aphotic zone. The Bolognano Fm. represents, due to its specific hydraulic properties (e.g. porosity and permeability), an outcropping analogue of worldwide common reservoirs (i.e. porous calcarenite deposits of a carbonate ramp formed by benthic foraminifera such as lepidocyclinids, nummulitids, red algae, corals). In the study area, several geological units of the Bolognano Fm. are characterized by abundant hydrocarbon (bitumen) occurrences infilled within the high-porosity of the cross-bedded calcarenites ascribed to the Chattian and Burdigalian interval. The geological field mapping of the area and the visualization of the geological data in a 3D environment show that the unit formed by mid-ramp calcarenites (Lepidocyclina calcarenites 2 unit, Chattian–Burdigalian) increases in thickness towards the NE (basinward) direction as a consequence of sediment shedding from inner ramp. Our study illustrates how the geological mapping and the visualization and analysis of geological data in a 3D environment of the northernmost sector of the Majella Mountain confirms depositional models of the Bolognano Formation and represents a valid tool for the characterization of the lateral stratigraphic relationships within this formation, and hence of its potential hydrocarbon occurrences.
  • Anisotropic grossular–andradite garnets: Evidence of two stage skarn evolution from Rudnik, Central Serbia

    Abstract: This paper presents LA-ICP-MS data for garnets from the Rudnik skarn deposit (Serbia), which range from Grs45–58Adr40–52Alm2–3 in the core and Adr70–97Grs2–29Sps1 in the rim displaying anisotropy and zoning. In spite of wide compositional variations the garnets near the end-member of andradite (Adr > 90) are generally isotropic. Fe-rich rims exhibit LREE depletion and flat HREE pattern with weak negative Eu anomaly, including higher As and W contents. On the other side, the Fe-poorer core shows flat REE pattern without any significant enrichment or depletion of REE, except higher amounts of trace elements, such as U, Th and Zr. Presence of sulphide minerals indicates reduction conditions and Eu divalent state. Different REE behaviour is conditioned by Eu2+ in reduction conditions. The observed variations in optical features and garnet chemistry are the results of their two-stage evolution. The first stage and period of garnet growth is probably buffered by mineral dissolution and reactions in the country rock. The second stage is related to hydrothermal activity when W and Fe were brought into the system probably by a boiling process in the volcanic event in the late Oligocene 23.9 Ma.
  • A Lower Miocene pyroclastic-fall deposit from the Bükk Foreland Volcanic Area, Northern Hungary: Clues for an eastward-located source

    Abstract: Detailed investigation of a Lower Miocene Plinian pyroclastic sequence that crops out in the Bükk Foreland Volcanic Area (BFVA) in Northern Hungary is presented here. The studied eruptive products are part of a ca. 50 metres thick pyroclastic succession comprising of a basal ignimbrite that is covered by stratified pyroclastic unit including a topmost ignimbrite (Mangó ignimbrite unit, part of the Lower Pyroclastic Complex). The investigated pyroclastic unit is part of the Mangó ignimbrite unit, and consists of a pyroclastic fallout deposit, a ground-surge deposit, and an ignim­brite, all indicating a complete Plinian eruption phase. This pyroclastic succession has been identified in three locations, which crops out along a ~20 km long, SW–NE transect in the BFVA (two in the western, and one in the eastern part). The pyroclastic rocks in these sites are correlated well on the basis of the lithologically and texturally similar layers and their identical field volcanological properties. The correlation is also supported by the paleomagnetic signature of the two ignimbrites (upper ignimbrite – declination: 275–302°, lower ignimbrite with overprint magnetization – declination: 320–334°). The paleomagnetic directions of the stratigraphically upper ignimbrite suggest that this sequence belongs to the oldest known pyroclastic rock assemblages of the BFVA (Lower Pyroclastic Complex, deposited between 18.5 and 21 Ma according to previously published K/Ar dating results in good agreement with paleomagnetic measurements). Based on proximal-to-distal variations in the grain size of the pyroclastic fallout deposit (with maximal thickness is 71 cm), a potential source region to the east (or northeast, or southeast) of the BFVA has been inferred in a relatively close distance (~5–15 km). The (north)eastward-located source region is also supported by comparison of the characteristics of the studied fallout deposit with the spatial distribution of selected Plinian fallout tephra from worldwide examples using their digitalized isopach maps.
  • Lower Miocene olistostromes and giant-olistoliths: A new interpretation of the Eocene Waschberg Limestone occurrences and consequences for the structural composition of the southern Waschberg–Ždánice Unit in Lower Austria

    Abstract: The Waschberg–Ždánice Unit links the Alpine and Carpathian orogens. Its complex structural and sedimentary structures lack a modern interpretation, particularly in the Austrian part. In recent years, the southern end of the Waschberg–Ždánice Unit has been geologically mapped in detail. Nine large occurrences (km-size) of the Waschberg Limestone, particularly at Waschberg, Michelberg, Praunsberg, and at some unnamed places continue into and strike in line with the widespread olistostromes. They are consequently interpreted as giant-olistoliths and represent products of submarine mass transport processes contemporaneous with the adjacent olistostromes. Signs for large-scale imbricate structures (repetitive sequences) or interpretation as tectonic klippen were not found. Based on the detailed geological mapping, some previously unknown structural elements are introduced, such as Haselbach Wedge and ”crunch-zone”. The Waschberg Limestone itself is an allochthonous mixed sediment (high density debrites and turbidites) that contains shallow water benthic (e.g., Nummulites) and deep-water planktic foraminifera of different age. Formation and final deposition of the Waschberg Limestone included sedimentation of Ypresian larger foraminifera and other biogenic grains in an Ypresian/basal Lutetian basin, detachment and transport towards the north-west, mixture with crystalline basement fragments and Flysch components in an Egerian or basal Eggenburgian foredeep, exposure on unstable slopes of the thrust front, and finally mobilization and basinward transport of olistostromes and Waschberg Limestone giant olistoliths during the Eggenburgian. The formation of olistostromes and giant-olistoliths may be indicative for the increased velocity or higher intensity of the thrusting processes during the early Miocene.
  • Specific green zonal silica nodules of serpentinite weathering: Unusual products of silicification in laterite-like residuum (Moldanubian Zone, Bohemian Massif)

    Abstract: Massive quartz–chalcedony ± opal nodules (“plasma” in gemology) represent a specific silica variety, which occurs in the laterite-like residues of pre-Miocene paleo-weathering of ultramafites in western Moravia (Moldanubian Zone, Bohemian Massif). These zonal silica nodules (ZSN) tend to have concentric texture with a dark green to green-brown core, pale green margin and a narrow white rim (outer surface zone). The most typical microscopic feature of ZSN is vermiform microstructure particularly in the two outer zones. Individual zones consist of micro- to non-crystalline SiO2 polymorphs with variable contents of H2O (quartz, chalcedony, moganite, opal-C/CT and opal-A). The predominant green colour is due to submicroscopic smectite pigment, while the brownish colour originated from decomposition of smectite to iron oxohydroxides. ZSN formed in subaerial, partially reducing conditions in the lower part of weathering crusts covering serpentinites. The whole process was preceded by component exchange (chloritization) along serpentinite – felsic rocks (granulite, migmatite, pegmatite veins) boundaries. The gradual silica migration and subsequent redistri­bution associated with the removal of aluminium, magnesium and iron led up to the formation of a zonal nodular texture dominated by SiO2 polymorphs. Newly formed minerals in micro-cavities and cracks of ZSN are represented by acces­sory pyrite and sporadic barite. Zonal silica nodules-bearing residues on serpentinites occur only in a narrow area which was originally covered by clay-sandy Miocene sediments of the Carpathian Foredeep in western Moravia. Probably late low-temperature fluid interaction between silicified serpentinite residuum (chlorite – montmorillonite saprolite) and marine sediments may be the main factor controlling formation of ZSN.